1. Field of the Invention
This invention is concerned with double layer capacitors and, more particularly, with double layer capacitors having polymeric electrolytes and methods for making such capacitors.
2. Description
Double layer capacitors are disclosed in prior U.S. patents including:
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In general, the above patents describe double layer capacitors which comprise a pair of polarization electrodes having a separating medium therebetween. The electrodes are composed of a solid and liquid phase and the electric double layer which characterizes these capacitors is formed at the interface between the solid and liquid (electrolyte) phases of the electrodes. The separating medium acts as an electronic insulator between the electrodes, but is sufficiently porous to permit ion migration therethrough.
The double layer capacitors disclosed in the prior art are half-cells assembled on either side of a separator element. Each half-cell comprises an electrode element, an enclosing cell wall for the electrode element and a conductor element contacting one end wall of the electrode element. The electrode is made up of high surface area carbon material associated with an electrolyte such as H.sub.2 SO.sub.4, at a concentration of, say, 25 wt. % H.sub.2 SO.sub.4. The high surface area or activated carbon used in the electrodes has a surface area in the range from 1400 to 1600 square meters per gram.
The use of a liquid electrolyte, of course, limits the operating temperature range of the capacitors. Further, the electrolyte/activated carbon mixture which is used to form the electrodes is a damp powder which is difficult to handle, particularly in accurately measuring quantities of the powder. Further, the damp powder does not easily fill all corners of the electrode mold volume under reasonable pressure. The difficulty increases when the capacitors must be made in miniature size and in thin configurations.
The following technical articles are directed to developments in the art which are relevant to the present invention:
"Lithium Conductors of Polyion Complexes Dispersed with LiClO.sub.4 and Their Application to Solid-State Batteries", by S. Toyota, T. Nogami and H. Mikawa, Solid State Ionics 13 (1984) 243-247, North-Holland, Amsterdam. A polyion complex dispersed with LiClO.sub.4 was used in pellet configuration to form capacitors in which the electrolyte pellet is sandwiched between two sheets of activated carbon fiber cloth. Also, polyethylene oxide dispersed with LiBF.sub.4 was used as a solid electrolyte in a battery between electrodes of lithium and activated carbon fiber.
"Solid State Micro Power Sources", by S. Sekido, Solid State Ionics 9 & 10 (1983) 777-782, North-Holland Publishing Company. Double layer capacitors utilizing either Cu.sup.+ or Ag.sup.+ ion conductor have a positive electrode of active charcoal and a negative electrode of either Cu or Ag with the electrodes separated by a solid electrolyte layer. While Li.sup.+ ion conductor is mentioned for use in rechargeable power sources, such power sources are stigmatized as impractical.
It will be appreciated that if means could be found to manufacture these double layer capacitors having high surface area electrodes so that they are substantially in the solid state when completed with good electrical properties, that many or all of the disadvantages of capacitors using liquid electrolytes would be avoided.